The present invention relates to an exhaust equipment member such as an exhaust manifold, a turbine housing, etc. for automobile engines, an internal combustion engine system using such an exhaust equipment member, and a method for producing such an exhaust equipment member.
Exhaust equipment members such as exhaust manifolds, turbine housings, etc. for automobiles are conventionally made of heat-resistant cast iron such as NI-RESIST cast iron (Nixe2x80x94Crxe2x80x94Cu austenitic cast iron), heat-resistant ferritic cast steel, etc. Though the NI-RESIST cast iron has relatively good high-temperature strength at an exhaust gas temperature of up to 900xc2x0 C., it does not have enough durability at a temperature of 900xc2x0 C. or higher. Also, the heat-resistant ferritic cast steel is poor in a high-temperature strength at an exhaust gas temperature of 950xc2x0 C. or higher.
There is a heat-resistant, austenitic cast steel as a material more resistant to a high temperature than heat-resistant cast iron such as NI-RESIST cast iron and heat-resistant ferritic cast steel. For instance, Japanese Patent Laid-Open No. 54-96418 discloses a heat-resistant, austenitic cast steel comprising by weight 0.1-1.5% of C, 0.5-5.0% of Si, less than 2.5% of Mn, 15-35% of Cr, and 8-45% of Ni, 0.5-3.0% of W, 0.2-5.0% of Nb, or further 0.5-2.0% of Mo and 0.05-0.25% of S, the balance being substantially Fe. This Japanese laid-open application shows in Examples a heat-resistant, austenitic cast steel having a composition comprising by weight 0.12-1.42% of C, 0.23-0.73% of Si, 0.77-0.83% of Mn, 0.87-1.62% of Mo, 24.8-25.3% of Cr, 19.6-20.3% of Ni, 0.86-1.6% of W, 0.21-1.33% of Nb, and 0.08-16% of S, the balance being substantially Fe. Because this cast steel contains S, it exhibits improved cuttability, a high-temperature tensile strength of 10.6-15.4 kg/mm2 at 1000xc2x0 C., and a weight loss by oxidation of 1.7-8.3 mg/(dm2xc2x7hr) at 900xc2x0 C.
The present applicant proposed heat-resistant, austenitic cast steels durable in use at a high temperature of 900xc2x0 C. or higher (Japanese Patent Laid-Open Nos. 5-5161 and 7-228948).
Japanese Patent Laid-Open No. 5-5161 discloses a heat-resistant, austenitic cast steel having a composition comprising by weight 0.20-0.60% of C, 2.00% or less of Si, 1.00% or less of Mn, 15-30% of Cr, 8-20% of Ni, 2-6% of W, 0.2-1.0% of Nb, and 0.001-0.01% of B, the balance being substantially Fe and inevitable impurities, which has excellent high-temperature strength even after subjected to repeated heat cycles of heating up to higher than 900xc2x0 C. and cooling, and an exhaust equipment member made of such heat-resistant austenitic cast steel. This Japanese laid-open application shows in EXAMPLE a composition comprising by weight 0.19-0.49% of C, 0.87-1.06% of Si, 0.46-0.59% of Mn, 18.82-28.20% of Cr, 8.26-18.84% of Ni, 2.02-5.03% of W, 0.28-0.98% of Nb, and 0.002-0.008% of B, the balance being substantially Fe and inevitable impurities, or further 0.49-0.55% of Mo and/or 4.50-18.74% of Co. EXAMPLES of this Japanese laid-open application show that the heat-resistant austenitic cast steel had a 0.2-% yield strength of 33-62 MPa, a tensile strength of 59-31 MPa and an elongation of 27-40% at 1050xc2x0 C. Also, they show that when the thermal fatigue life of this austenitic cast steel was measured on a round rod test piece having a gauge length of 20 mm and a diameter of 10 mm in the gauge length under the conditions of the lowest heating temperature of 150xc2x0 C., the highest heating temperature of 1000xc2x0 C., and each one cycle of 12 minutes in a state where the elongation and shrink of the test piece by heating were mechanically completely constrained, the number of cycles was 88-195 until the thermal fatigue failure took place. Further, they show that the weight loss by oxidation after kept at 1000xc2x0 C. for 200 hours was 15-50 mg/cm2.
Japanese Patent Laid-Open No. 7-228948 discloses a heat-resistant, austenitic cast steel with excellent castability and cuttability having a composition comprising by weight 0.2-1.0% of C, 2% or less of Si, 2% or less of Mn, 15-30% of Cr, 8-20% of Ni, 1-6% of W, 0.5-6% of Nb, 0.01-0.3% of N, and 0.01-0.5% of S, Cxe2x80x94Nb/8 being 0.05-0.6%, and the balance being substantially Fe and inevitable impurities, and an exhaust equipment member made of such austenitic cast steel. This Japanese laid-open application shows in EXAMPLE a composition comprising by weight 0.21-0.80% of C, 0.52-1.11% of Si, 0.51-1.05% of Mn, 16.55-21.02% of Cr, 8.45-18.55% of Ni, 1.02-5.80% of W, 0.68-6.95% of Nb, 0.03-0.14% of N, and 0.03-0.41% of S, Cxe2x80x94Nb/8 being 0.12-0.58%, and the balance being substantially Fe and inevitable impurities. The heat-resistant austenitic cast steel in this EXAMPLE had a 0.2-% yield strength of 55-80 MPa, a tensile strength of 62-125 MPa and an elongation of 26-75% at 1000xc2x0 C. Also, when the thermal fatigue life of this austenitic cast steel was measured on a round rod test piece having a gauge length of 25 mm and a diameter of 10 mm in the gauge length under the conditions of the lowest heating temperature of 150xc2x0 C., the highest heating temperature of 1000xc2x0 C., and each one cycle of 12 minutes in a state where the elongation and shrink of the test piece by heating were mechanically completely constrained, the number of cycles was 145-210 until the thermal fatigue failure took place. Further, it exhibited a weight loss by oxidation of 18-50 mg/cm2 when kept in the air at 1000xc2x0 C. for 200 hours.
In most automobile engines, gasoline is mixed with air in an intake manifold or a collector as an air-intake member and then supplied to a combustion chamber of the engine. In this structure, if gasoline or a mixture of gasoline and air leaks from the intake manifold or the collector by the collision of an automobile, it may be ignited. To prevent such an accident, air-intake members such as an intake manifold or a collector are connected to the engine on the rear side, while exhaust equipment members such as an exhaust manifold and a turbine housing are connected to the engine on the front side.
Recently, further reduction of an exhaust gas and improvement in fuel efficiency are increasingly demanded for the purpose of maintaining global environment. Thus, progress has been achieved in increase in the output of engines and the combustion temperature, resulting in the development and wide spreading of so-called direct-injection engines having combustion chambers into which gasoline is directly injected. In this direct-injection engine, because gasoline is directly introduced into a combustion chamber from a fuel tank, only the slightest amount of gasoline would leak if the automobile collided, resulting in little likelihood that the collision leads to a large accident. Accordingly, instead of the conventional arrangement that the exhaust equipment members such as an exhaust manifold and a turbine housing are disposed in front of the engine while the air-intake members such as an intake manifold or a collector are disposed on the rear side of the engine, the air-intake members may be disposed in front of the engine to supply a cooled air to the combustion chamber of the engine, while the exhaust equipment members are disposed on the rear side of the engine, so that they are directly connected to an exhaust gas-purifying apparatus to improve the initial performance of an exhaust gas-purifying catalyst in the exhaust gas-purifying apparatus.
When the exhaust equipment members such as an exhaust manifold and a turbine housing are disposed on the rear side of the engine, the surface temperatures of the exhaust equipment members are elevated because the exhaust equipment members are less likely to be brought into contact with the wind during the driving of an automobile. Thus, the exhaust equipment members need high durability at a high temperature.
The exhaust equipment members such as an exhaust manifold and a turbine housing are presently required to have enough durability to an exhaust gas at temperatures exceeding 1000xc2x0 C., or near 1050xc2x0 C., or further near 1100xc2x0 C. Further, to ensure the initial performance of an exhaust gas-purifying catalyst at the time of starting the engine, the exhaust equipment members should be as thin as possible.
The heat-resistant, austenitic cast steel disclosed in Japanese Patent Laid-Open No. 54-96418 exhibits a weight loss by oxidation of 1.7-8.3 mg/(dm2xc2x7hr) at 900xc2x0 C. and a tensile strength of 10.6-15.4 kg/mm2 at 1000xc2x0 C. Also, the heat-resistant, austenitic cast steel disclosed in Japanese Patent Laid-Open No. 5-5161 exhibits a weight loss by oxidation of 15-50 mg/cm2 after kept at 1000xc2x0 C. for 200 hours. Further, the heat-resistant, austenitic cast steel disclosed in Japanese Patent Laid-Open No. 7-228948 exhibits a weight loss by oxidation of 18-50 mg/cm2 after kept at 1000xc2x0 C. for 200 hours. However, exhaust equipment members exposed to an exhaust gas at a temperature exceeding 1000xc2x0 C. are neither disclosed nor suggested in any of these Japanese patents.
Accordingly, an object of the present invention is to provide an exhaust equipment member having excellent durability even when exposed to an exhaust gas at a temperature exceeding 1000xc2x0 C. or near 1050xc2x0 C. or further near 1100xc2x0 C., which may be thin and disposed on the rear side of an engine to improve the initial performance of an exhaust gas-purifying catalyst.
Another object of the present invention is to provide an internal combustion engine system comprising such an exhaust equipment member.
A further object of the present invention is to provide a method for producing such an exhaust equipment member.
The inventors have investigated how to improve high-temperature characteristics such as oxidation resistance and thermal fatigue life by changing the amounts of C, Cr, Ni, S, W, Nb, etc. added to a basic composition of a heat-resistant, high-Cr, high-Ni, austenitic cast steel. As a result, they have found that not only a high-temperature strength but also oxidation resistance are important factors to improve the durability of the exhaust equipment member exposed to an exhaust gas at a temperature exceeding 1000xc2x0 C. Specifically, when the exhaust equipment member is oxidized by exposure to a high-temperature exhaust gas, fine cracks are generated thereon, functioning as starting sites of oxidation, resulting in further generation of fine cracks. This mechanism occurs repeatedly to generate large cracks, which determines the durability of the exhaust equipment member.
To suppress the oxidation of the exhaust equipment member as much as possible, it has been found that the composition of the heat-resistant, high-Cr, high-Ni, austenitic cast steel having weight ratios of Cr/Ni of 1.0-1.5 and Mn/S of 5 or more, particularly a weight ratio of Cr/Ni should be optimized, thereby precipitating fine carbide particles based on chromium in the austenitic matrix to improve oxidation resistance. Thus, it is possible to obtain an exhaust equipment member having an exhaust gas path portion at least partially having a thickness of 5 mm or less, and having excellent durability even when exposed to an exhaust gas at a temperature exceeding 1000xc2x0 C. or near 1050xc2x0 C. or further near 1100xc2x0 C. It has also been found that by vacuum casting with a sand mold, a melt of the heat-resistant, high-Cr, high-Ni, austenitic cast steel having weight ratios of Cr/Ni of 1.0-1.5 and Mn/S of 5 or more can flow well to form an exhaust gas path portion at least partially as thin as 5 mm or less. It has further been found that by disposing the above exhaust equipment member on the rear side of the engine, an exhaust gas-purifying catalyst arranged downstream of the exhaust equipment member can exhibit improved initial performance. The present invention has been completed based upon these findings.
Thus, the exhaust equipment member according to the first embodiment of the present invention has an exhaust gas path portion at least partially having a thickness of 5 mm or less, the exhaust equipment member being made of a heat-resistant, high-Cr, high-Ni, austenitic cast steel having weight ratios of Cr/Ni of 1.0-1.5 and Mn/S of 5 or more, with a weight loss by oxidation of 50 mg/cm2 or less when kept in the air at 1010xc2x0 C. for 200 hours.
The exhaust equipment according to the second embodiment of the present invention has an exhaust gas path portion at least partially having a thickness of 5 mm or less, the exhaust equipment member being made of a heat-resistant, high-Cr, high-Ni, austenitic cast steel having weight ratios of Cr/Ni of 1.0-1.5 and Mn/S of 5 or more, with a weight loss by oxidation of 100 mg/cm2 or less when kept in the air at 1050xc2x0 C. for 200 hours.
The exhaust equipment member according to the third embodiment of the present invention has an exhaust gas path portion at least partially having a thickness of 5 mm or less, the exhaust equipment member being made of a heat-resistant, high-Cr, high-Ni, austenitic cast steel having weight ratios of Cr/Ni of 1.0-1.5 and Mn/S of 5 or more, with a weight loss by oxidation of 200 mg/cm2 or less when kept in the air at 1100xc2x0 C. for 200 hours.
The exhaust equipment member according to the fourth embodiment of the present invention has an exhaust gas path portion at least partially having a thickness of 5 mm or less, the exhaust equipment member being made of a heat-resistance, high-Cr, high-Ni, austenitic cast steel having weight ratios of Cr/Ni of 1.0-1.5 and Mn/S of 5 or more, with a weight loss by oxidation of 50 mg/cm2 or less when kept in the air at 1010xc2x0 C. for 200 hours, and 100 mg/cm2 or less when kept in the air at 1050xc2x0 C. for 200 hours.
The exhaust equipment member according to the fifth embodiment of the present invention has an exhaust gas path portion at least partially having a thickness of 5 mm or less, the exhaust equipment member being made of a heat-resistant, high-Cr, high-Ni, austenitic cast steel having weight ratios of Cr/Ni of 1.0-1.5 and Mn/S of 5 or more, with a weight loss by oxidation of 100 mg/cm2 or less when kept in the air at 1050xc2x0 C. for 200 hours, and 200 mg/cm2 or less when kept in the air at 1100xc2x0 C. for 200 hours.
The exhaust equipment member according to the sixth embodiment of the present invention has an exhaust gas path portion at least partially having a thickness of 5 mm or less, the exhaust equipment member being made of a heat-resistant, high-Cr, high-Ni, austenitic cast steel having weight ratios of Cr/Ni of 1.0-1.5 and Mn/S of 5 or more, with a weight loss by oxidation of 50 mg/cm2 or less when kept in the air at 1010xc2x0 C. for 200 hours, 100 mg/cm2 or less when kept in the air at 1050xc2x0 C. for 200 hours, and 200 mg/cm2 or less when kept in the air at 1100xc2x0 C. for 200 hours.
Any of the above exhaust equipment members preferably has a thermal fatigue life of 200 cycles or more in a thermal fatigue test in which heating and cooling are repeated under the conditions of the highest heating temperature of 1000xc2x0 C., a temperature amplitude of 800xc2x0 C. or more and a constraint ratio of 0.25.
Any of the above exhaust equipment members preferably has a thermal fatigue life of 100 cycles or more in a thermal fatigue test in which heating and cooling are repeated under the conditions of the highest heating temperature of 1000xc2x0 C., a temperature amplitude of 800xc2x0 C. or more and a constraint ratio of 0.5.
In any of the above exhaust equipment members, the heat-resistant, high-Cr, high-Ni, austenitic cast steel having weight ratios of Cr/Ni of 1.0-1.5 and Mn/S of 5 or more preferably has a composition by weight comprising 0.2-1.0% of C, 2% or less of Si, 2% or less of Mn, 0.04% or less of P, 0.05-0.25% of 5, 20-30% of Cr, and 16-30% of Ni, the balance being substantially Fe and inevitable impurities. The more preferred composition of the heat-resistant, high-Cr, high-Ni, austenitic cast steel comprises by weight 0.3-0.6% of C, 0.2-1.0% of Si, 0.8-1.5% of Mn, 0.04% or less of P, 0.12-0.20% of S, 23-27% of Cr, and 18-22% of Ni, the balance being substantially Fe and inevitable impurities.
In a preferred embodiment, the heat-resistant, high-Cr, high-Ni, austenitic cast steel having weight ratios of Cr/Ni of 1.0-1.5 and Mn/S of 5 or more further comprises 1-4%, more preferably 2.7-3.3%, of Wand/or more than 1% and 4% or less, more preferably 1.8-2.2%, of Nb by weight. Further preferably, the heat-resistant, high-Cr, high-Ni, austenitic cast steel having weight ratios of Cr/Ni of 1.0-1.5 and Mn/S of 5 or more further comprises Mo at a ratio of W=2 Mo. A weight ratio of Cr/Ni is preferably 1.0-1.5. A weight ratio of Mn/S is preferably 5 or more, thereby containing sulfide particles including manganese sulfide.
The heat-resistant, high-Cr, high-Ni, austenitic cast steel having weight ratios of Cr/Ni of 1.0-1.5 and Mn/S of 5 or more preferably has a structure of an austenitic matrix in which fine carbide particles based on chromium are uniformly precipitated.
The exhaust equipment member may be an exhaust manifold, a turbine housing, an exhaust manifold integral with a turbine housing, a catalyst case, or an exhaust manifold integral with a catalyst case.
The internal combustion engine system according to an embodiment of the present invention comprises an engine, an air-intake member connected to the front side of the engine, and the above-described exhaust equipment member connected to the rear side of the engine, wherein at least an exhaust manifold is directly connected to an exhaust gas-purifying apparatus.
The method for producing an exhaust equipment member having an exhaust gas path portion at least partially having a thickness of 5 mm or less according to an embodiment of the present invention comprises the steps of (1) preparing a sand mold having a cavity for receiving a melt of an heat-resistant, high-Cr, high-Ni, austenitic cast steel for forming the exhaust equipment member, a sprue connected to the cavity via a gate, and an air-permeable portion close to a part of the cavity into which the melt flows substantially last and apart from the gate, (2) evacuating the cavity through the air-permeable portion of the sand mold; (3) pouring the melt of an heat-resistant, high-Cr, high-Ni, austenitic cast steel having weight ratios of Cr/Ni of 1.0-1.5 and Mn/S of 5 or more having a composition by weight comprising 0.2-1.0% of C, 2% or less of Si, 2% or less of Mn, 0.04% or less of P, 0.05-0.25% of 5, 20-30% of Cr, and 16-30% of Ni, the balance being substantially Fe and inevitable impurities, into the cavity for casting; and (4) heat-treating the resultant casting, so that it has a structure of an austenitic matrix in which fine carbide particles based on chromium and having an average particle size of 10 xcexcm or less are uniformly precipitated.